研究领域

Physical & Theoretical Chemistry

Summary of Research Interests My research interests focus upon the development and application of diode laser-based spectroscopic techniques to a variety of fundamental and applied problems in gas phase chemistry. The research seeks to employ cutting edge optical material technology to produce continuous wave, narrow band, high power, single mode laser sources operating in the uv and mid-IR spectral regions and to use such radiation in conjunction with sensitive time-resolved absorption methods for novel experiments in reaction dynamics, plasmas diagnostics and aeronomy. Frequency up and down conversion in periodically poled materials High performance, low noise single mode diode lasers are commercially available in the wavelength range 635nm to 1.7µm; this region however is not very interesting for diagnostic purposes as most electronic transitions lie at shorter wavelengths (uv), and fundamental vibrational transitions are at considerably longer wavelengths (mid-IR). Such wavelengths can however be generated by nonlinear optical methods. These methods are generally inefficient when using standard birefringent crystals in conjunction with continuous wave (cw) diode lasers and my research circumvents this by employing an alternative methodology which relies upon nonlinear interactions in novel periodically poled crystals and waveguides. These ferroelectric materials offer a sufficiently enhanced nonlinearity compared to conventional birefringent crystals such that enhancement cavities are not required and thus they are robust sources of high power uv or mid-IR radiation that has (inherently) low amplitude noise. Currently, uv generation is carried out using periodically poled magnesium oxide doped lithium tantalate, (PPMgOLT), to produce radiation down to 325nm while production of mid-IR radiation (approx. =3.3µm) is achieved by difference frequency generation with a YAG laser and a selection of diode lasers (operating in the telecommunications band(s)) in periodically poled lithium niobate (PPLN). (i) Reaction dynamics The objective of this research area is to study the fundamental vector properties of chemical processes by developing and applying sensitive diode laser based absorption techniques. An example of a vector property is the relationship between the velocity vector of a reaction product and its angular momentum vector and such properties have been found to be very sensitive to the topography of the potential energy surface. Consequently the measurement of vector correlations constitute a stringent test for ab-initio calculations and provide information on the photodynamics, the multiple potential energy surfaces accessed, and non-adiabatic processes during dissociation. (ii) Plasma diagnostics Quantitative measurements of the concentrations of trace gas species in plasmas are vital for understanding and directing research in this applied area. Diode lasers in combination with modulation spectroscopies and/or cavity enhanced methods are sensitive enough to measure physico-chemical properties such as the absolute number densities, translational, internal temperatures, field effects and the electron energy distribution as a function of the plasma operating conditions. (iii) Aeronomy Many trace species play important roles in key processes that determine the equilibrium and composition of the atmosphere, OH for example (whose concentration is typically 4 – 6 parts per trillion) essentially controls the daytime oxidative capacity of the atmosphere. In order to fully understand the chemistry occurring in our atmosphere it is necessary to be able to measure the concentrations of these trace species both accurately and quickly. Cavity enhanced spectroscopies in conjunction with diode lasers provide a robust, sensitive and accurate method for making quantitative measurements of such species in both clean and polluted atmospheres.

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Enhancing the sensitivity of mid-IR quantum cascade laser-based cavity-enhanced absorption spectroscopy using RF current perturbation K. M. Manfred, J. M. R. Kirkbride, L. Ciaffoni, R. Peverall, G. A. D. Ritchie, Opt. Lett. 39, 6811-6814 (2014) DOI: 10.1364/OL.39.006811 RF noise induced laser perturbation for improving the performance of non-resonant cavity enhanced absorption spectroscopy L. Ciaffoni, J. Couper, G. Hancock, R. Peverall, P. A. Robbins, G.A.D. Ritchie, Opt. Exp. 22, 17030-17039 (2014) DOI:10.1364/OE.22.017030 Using a DS-DBR laser for widely tunable near-infrared cavity ring-down spectroscopy K.E. Whittaker, L. Ciaffoni, G. Hancock, P.L. Hurst, R. Peverall, G.A.D. Ritchie, Appl. Phys. B. 116, 157-168 (2013) DOI 10.1007/s00340-013-5667-z Optical trapping and spectroscopic characterisation of ionic liquid solutions L.J. Moore, M.D. Summers, G.A.D. Ritchie, Phys. Chem. Chem. Phys. 15, 13489-13498 (2013) DOI 10.1039/C3CP50895A Sub-Doppler spectroscopy with an external cavity quantum cascade laser R.J. Walker, J. Kirkbride, J-P.H. van Helden, D. Weidmann, G.A.D. Ritchie, App. Phys. B 112, 159-167 (2013) DOI 10.1007/s00340-013-5410-9 (2013). Vector correlations in the O2 (a1Δg, v = 1) fragment formed in the 265 nm photodissociation of ozone G. Hancock, G.A.D Ritchie, T.R. Sharples, Mol. Phys. DOI:10.1080/00268976.2013.780104 (2013) Predissociation dynamics of the C 3Pg Rydberg state of molecular oxygen A. Gilchrist and G.A.D. Ritchie, J. Chem. Phys 138 104320 (2013) Coherent Transient Spectroscopy with Continuous Wave Quantum Cascade Lasers J.M.R. Kirkbride, S.E. Causier, E.A. McCormack, D. Weidmann, G.A.D. Ritchie, Phys. Chem. Chem. Phys. 15, 2684-2691 (2013) DOI: 10.1039/C2CP44116k (2013). Demonstration of a Mid-Infrared Cavity Enhanced Absorption Spectroscopy for Breath Acetone Detection L. Ciaffoni, G. Hancock, J. J. Harrison, J. H. van Helden, C. E. Langley, R. Peverall, G.A.D. Ritchie, S. Wood, Analytical Chemistry, 85, 846-850 (2013) DOI: 10.1021/ac3031465 Demonstration of a widely tunable digital supermode distributed Bragg reflector laser as a versatile source for near-infrared spectroscopy, L. Ciaffoni, G. Hancock, P.L. Hurst, M. Kingston, C.E. Langley, R. Peverall, G.A.D. Ritchie, K.E. Whittaker, Appl. Phys. B 110, 139-145 (2013) DOI: 10.1007/s00340-011-4869-5 Single aerosol trapping with an annular beam: improved particle localisation, R. D. Dear, D. R. Burnham, M. D. Summers, D. McGloin, G. A. D. Ritchie, Phys. Chem. Chem. Phys. 14, 15826, (2012) DOI: 10.1039/C2CP42925J A DFG-based cavity ring-down spectrometer for trace gas sensing in the mid-infrared, K.E. Whittaker, L. Ciaffoni, G. Hancock, R. Peverall, G.A.D. Ritchie, Appl. Phys. B 109, 333-343 (2012) DOI: 10.1007/s00340-012-5150-2 Combining a DS-DBR laser with QPM-DFG for mid-infrared spectroscopy, K.E. Whittaker, L. Ciaffoni, G. Hancock, M. Islam, R. Peverall, G.A.D. Ritchie, Appl. Phys. B 109, 423-432 (2012) DOI: 10.1007/s00340-012-5071-0 UV photodissociation dynamics of iodobenzene: effects of fluorination, D. Murdock, M. B. Crow, G. A. D. Ritchie, M. N. R. Ashfold, J. Chem. Phys. 136, 124313, (2012) DOI: 10.1063/1.3696892 Noise-immune cavity-enhanced optical heterodyne detection of HO2 in the near-infrared, C. L. Bell, J-P. van Helden, G. Hancock, N. J. van Leeuwen, R. Peverall, G. A. D. Ritchie, J.. Phys. Chem. A, 116, 5090, (2012) DOI: 10.1021/jp301038r Electronic polarization effects in the photodissociation of Cl2, E. Campbell, A. B. Alekseyev, G. Balint-Kurit, M. Brouard, A. Brown, R. Buenker, R. Cireasa, A. J. Gilchrist, A. Johnsen, D. Kokh, S. Lucas, G. A. D. Ritchie, T. R. Sharples, B. Winter, J. Chem. Phys. 136, 164311 (2012), DOI: 10.1063/1.4704830 Directed assembly of optically bound matter, M.D. Summers, R.D. Dear, J.M. Taylor, G.A.D. Ritchie, Opt. Express 20, 1001, (2012). Population transfer and rapid passage effects in a low pressure gas using a continuous wave quantum cascade laser, E.A. McCormack, H.S. Lowth, M.T. Bell, D. Weidmann, G.A.D. Ritchie, J. Chem. Phys. 137, 034306, (2012) Laser spectroscopy on volatile sulfur compounds: possibilities for breath, L. Ciaffoni, R. Peverall, G.A.D. Ritchie, J. Breath Res. 5, 024002, (2012). Laser-based absorption spectroscopy as a technique for rapid in-line analysis of respired gas concentrations of oxygen and carbon dioxide, B. Cummings, M. Hamilton, L. Ciaffoni, T. R. Pragnell, R. Peverall, G. A. D. Ritchie, G. Hancock, P. A. Robbins, J. Appl. Phys. 111, 303 (2011). Rapid passage signals from a vibrationally excited target molecule: a pump and probe experiment with continuous wave quantum cascade lasers, R. J. Walker, J. H. van Helden, J. Kirkbride, E. A. McCormack, M. T. Bell, D. Weidmann, G. A. D. Ritchie, Opt. Lett. 36, 4725, (2011).